The present invention relates generally to the field of digital communications. More specifically, the present invention relates to transmitter power amplifier linearization through the use of predistortion.
Power amplifiers are one of the most expensive and most power-consuming devices in communication systems. Digital predistortion is a technique that both reduces power amplifier cost while improving efficiency. Predistortion refers to distortion intentionally applied to a communication signal prior to amplification in a power amplifier. The distortion is configured to be the inverse of unwanted distortion introduced by the power amplifier, so that the resulting amplified communication signal comes out as nearly linear as possible.
With predistortion the power linearity is improved and extended so that the power amplifier can be operated at higher power. This means that a lower-power, lower-cost linearized power amplifier can be used in place of a higher-power, higher-cost power amplifier. Furthermore, the linearized power amplifier operates more efficiently, and a lower-power amplifier operating more efficiently consumes substantially less power than an inefficient higher-power amplifier. Moreover, these benefits are even more pronounced for multicarrier and CDMA applications where peak-to-average ratios tend to be large.
In general, gain and phase transfer characteristics of a typical power amplifier change as a function of the magnitude of the communication signal being amplified. In particular, gain tends to droop and phase shift tends to increase as communication signal magnitude approaches a saturation point for the power amplifier. Accordingly, a typical linearizer will amplify the communication signal by an amount which is a function of magnitude to compensate for gain droop, and apply an opposing-polarity phase shift as a function of magnitude to compensate for the power amplifier-induced phase shift.
However, a need exists to apply linearization so that a communication signal amplified in a power amplifier is as precisely linear as possible to achieve the greatest benefits. A variety of power amplifier memory effects make the generation of highly effective predistortion signals difficult. In general, memory effects refer to tendencies of power amplifiers to act differently in one set of circumstances than in another. For example, the gain and phase transfer characteristics of a power amplifier may vary as a function of frequency, instantaneous power variation, amplifier bias conditions, temperature, and component aging.
Frequency and bias-related memory effects tend to demonstrate short time constants, typically on the order of a few unit intervals. A unit interval refers to the baseband signal sampling period or approximately the inverse of the bandwidth or data rate of the modulated RF signal. These short-time-constant memory effects may be addressed by power amplifier circuit design and also by linearizer design. One effective technique for addressing the problem of short-time-constant memory effects in a linearizer applies different translation functions to different unit intervals in much the same manner that a digital filter applies different coefficients to different samples. Each translation function requires its own look-up table, with each look-up table having in-phase and quadrature components. Unfortunately, a large number of look-up tables is required.
Memory effects due to component aging and other factors can demonstrate extremely long time constants, often greater than hundreds of millions of unit intervals. These extremely-long-time-constant memory effects may be addressed by employing a trainer that monitors the power amplifier output and, knowing the linearizer translation functions being applied, calculates a more accurate transfer function for the power amplifier and/or a more accurate linearizer translation function. The calculations performed by the trainer tend to be computationally intense, but can be performed through appropriate software programs running on a computer quickly enough to track an extremely-long-time-constant memory effect. Typically, look-up tables that implement translation functions are updated as needed to track the extremely-long-time-constant memory effect.
A problem exists in tracking memory effects that exhibit time constants between the short and the extremely long time constants. These memory effects are referred to as long time constants herein. A thermal memory effect is an example of a long-time-constant memory effect. Unlike the short-time-constant memory effects that tend to be related to the signal bandwidth or modulation data rate, the long-time-constant memory effects, such as the thermal memory effect, tend to be physically related. The transfer characteristics of a power amplifier change as a function of the temperature of the power amplifier, including the semiconductor structures of the components that form the power amplifier. This temperature is a function of the ambient temperature and of self-heating due to the power level at which the amplified communication signal is generated. Temperature changes tend to cause changes in power amplifier performance more quickly than the extremely-long-time-constant memory effects, but less quickly than short-time-constant memory effects. In a typical power amplifier, time constants in the range of tens of microseconds may be observed for power amplifier transfer characteristics when shifting between low and high input power levels due to self-heating in the power amplifier.
The short-time-constant memory effect solutions of applying multiple linearizing translation functions to different unit intervals of a communication signal are unsuitable for addressing long-time-constant memory effects. The undesirably large number of look-up tables used to apply correction over just a few unit intervals would be impractical when extended over the hundreds or thousands of unit intervals that characterize a long-time-constant memory effect. Moreover, the impracticability of this technique becomes exacerbated when applied to high speed data because of the higher power and processing complexities of high-throughput applications.
Likewise, the extremely-long-time-constant memory effect solutions of updating look-up tables as needed to track power amplifier performance changes are unsuitable for addressing long-time-constant memory effects. The training function is too computationally complex to be completed sufficiently fast to track thermal effects in a practical manner, and the data transfer requirements for updating entire look-up tables in sufficient time to track thermal effects are too great.
It is an advantage of the present invention that an improved power amplifier linearizer that compensates for thermal memory effects and method therefor are provided.
Another advantage of the present invention is that a power amplifier linearizer and method track power amplifier transfer function changes having long time constants without requiring an undesirably large number of look-up tables.
Another advantage of the present invention is that a power amplifier linearizer and method are suitable for use in connection with high speed data, where unit intervals tend to be less than a few microseconds.
Another advantage of the present invention is that a power amplifier linearizer and method are provided that use internal predistortion translation functions augmented by an external thermal modeler.
These and other advantages are realized in one form by an improved power amplifier linearizer which compensates for thermal memory effects of a power amplifier and includes first and second predistortion circuits, a temperature indicator, a combiner and a scaling circuit. The first predistortion circuit generates a first predistortion signal in response to a data-conveying signal, where the first predistortion signal is associated with a first temperature for the power amplifier. The second predistortion circuit generates a second predistortion signal in response to the data-conveying signal, where the second predistortion signal is associated with a second temperature for the power amplifier. The temperature indicator generates a temperature signal which correlates with temperatures experienced by the power amplifier. The combiner couples to the first and second predistortion circuits and the temperature indicator. The combiner produces a temperature compensated predistortion signal in response to the first predistortion signal, the second predistortion signal, and the temperature signal. The scaling circuit couples to the combiner and is adapted to receive the data-conveying signal. The scaling circuit scales the data-conveying signal in response to the temperature-compensated predistortion signal.